The JPEG scheme recommended by the ISO and ITU-T is being widely used as a method of compressively encoding images captured in digital cameras and the like. In accordance with the JPEG scheme, encoding is carried out using a discrete cosine transform.
At present, JPEG 2000 is being studied as a next-generation compression scheme. Though the details relating to JPEG 2000 are not described here, a feature of this scheme is that a discrete wavelet transform is used instead of the discrete cosine transform.
However, a problem with encoding using a wavelet transform is that the time needed for compression (encoding) and expansion (decoding) is greater than that with encoding using the discrete cosine transform. A further problem is that an apparatus not having means for expanding image data that has been compressively encoded using a wavelet transform is incapable of displaying an image that has been compressively encoded using the wavelet transform.